I think those who do this would put the compatibility label on the back, so that no other labelling is covered up. The adhesive used should be tested to ensure that the label does not fall off between the temperatures at which the blood is stored or at the temperature at which the blood is transfused (and, in my experience, it is harder to get the labels off than it is to get a senior member of staff to buy a round of drinks!), but you do have to ensure that the adhesive is of a type that does not leech through the plastic into the unit itself (seriously - there are some that do)!
I think those who do this would put the compatibility label on the back, so that no other labelling is covered up. The adhesive used should be tested to ensure that the label does not fall off between the temperatures at which the blood is stored or at the temperature at which the blood is transfused (and, in my experience, it is harder to get the labels off than it is to get a senior member of staff to buy a round of drinks!), but you do have to ensure that the adhesive is of a type that does not leech through the plastic into the unit itself (seriously - there are some that do)!